1. Field of the Invention
This invention generally relates to a drive controller for a stepping motor and more particularly to a drive controller for a stepping motor whose rotation direction is required to be reversed.
2. Description of the Related Art
In a general serial printer, a stepping motor is used to set the print head to a desired position. The print head is driven by rotation of the stepping motor and moves in a forward or backward direction according to the rotation direction of the stepping motor. The stepping motor has a plurality of exciting coils arranged around the rotor and is rotated by sequentially exciting the exciting coils two at one time as an exciting phase. At this time, the rotation angle of the stepping motor is determined by the exciting phase. The feature of the stepping motor is that the rotation angle can be precisely set and the speed thereof can be easily controlled, and therefore the print head can be set to the printing position with extremely small error.
Now, the drive controller of the stepping motor is explained. Generally, the drive controller includes a signal generator for generating a motor control signal to rotate the stepping motor by a preset rotation angle and a motor driver for shifting the exciting phase according to the motor control signal to supply an exciting current to two exciting coils of each exciting phase. Generally, the signal generator and motor driver are connected to each other via a cable. In a case where the stepping motor has exciting coils A, B, C and D, for example, the drive controller sequentially excites the exciting coil pairs AB, BC, CD and DA as respective exciting phases. The exciting phase is shifted in the order of the exciting coil pairs AB, BC, CD and DA when the rotation direction of the stepping motor is set in the clockwise direction and shifted in the order of the exciting coil pairs DA, CD, BC and AB when the rotation direction of the stepping motor is set in the counterclockwise direction. The stepping motor rotates by one step angle each time the exciting phase is shifted. The rotation speed of the stepping motor is determined by the interval between the exciting operations.
FIGS. 1 and 2 show an example of a conventional drive controller. In the drive controller shown in FIG. 1, a signal generator 3 generates a 4-bit motor control signal for specifying a next exciting phase at an exciting timing. The first to fourth bits of the motor control signal are respectively allotted to the exciting coils A, B, C and D and two of them are selectively set to "1" at the exciting timing. The motor driver 2 supplies an exciting current to the exciting coil pair AB, BC, CD or DA of the stepping motor 1 according to a control signal "1100", "0110", "0011" or "1001". In the drive controller shown in FIG. 2, a signal generator 3 generates a 2-bit motor control signal for specifying the rotation direction of the stepping motor 1 at an exciting timing. The first and second bits of the control signal are respectively allotted to the clockwise direction and counterclockwise direction and one of them is selectively set to "1" at the exciting timing. The motor driver 2 holds information on the current exciting phase of the stepping motor 1 and supplies an exciting current to an exciting coil pair of a next exciting phase in the clockwise direction or counterclockwise direction respectively corresponding to the control signal "10" or "01".
Recently, the demand for small and inexpensive printers has significantly increased. In this type of printer, it is preferable to set the number of bits of a motor control signal to be transmitted via a cable 4 as small as possible. For example, since the motor control signal of the drive controller shown in FIG. 2 is smaller in the number of bits than that of the drive controller shown in FIG. 1, the cable can be relatively easily laid and extended in the printer and can be connected to the signal generator 3 and motor driver 2 by use of a small connector. In the prior art, the number of bits of the motor control signal cannot be made smaller than that of the drive controller shown in FIG. 2.